1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member (which may, for example, be a photoconductive drum); an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
In known electrophotographic image forming apparatuses, toner images of yellow, magenta, cyan, and black are formed on photosensitive drums of the respective colors. The image forming apparatus using an intermediate transfer method employs a belt-type intermediate transfer member (hereinafter refers to simply as intermediate transfer belt) formed into an endless loop that contacts the photosensitive drums, forming a primary transfer nip therebetween. In the primary transfer nip, a toner image formed on the photosensitive drum is transferred primarily onto the intermediate transfer belt. This process is known as a “primary transfer process”.
A secondary transfer roller contacts a peripheral surface of the intermediate transfer belt, forming a secondary transfer nip therebetween, so that the toner image on the intermediate transfer belt is secondarily transferred onto a recording medium in a process known as “secondary transfer process”. A secondary transfer opposed roller is disposed inside the loop formed by the intermediate transfer belt, facing the secondary transfer roller with the intermediate transfer belt interposed therebetween. The secondary transfer opposed roller disposed inside the loop of the intermediate transfer belt is grounded. By contrast, the secondary transfer roller disposed outside the loop is supplied with a secondary transfer bias. With this configuration, a secondary transfer electric field causing the toner image to move electrostatically from the secondary transfer opposed roller side to secondary transfer roller side is formed. The toner image on the intermediate transfer belt is transferred secondarily onto a recording medium fed to the secondary transfer nip in appropriate timing such that the recording medium is aligned with the toner image formed on the intermediate transfer belt.
When using a recording medium having a coarse surface such as Japanese paper, a pattern of light and dark according to the surface condition of the recording medium appears in an output image. More specifically, toner does not transfer well to such embossed surfaces, in particular recessed portions of the surface. This inadequate transfer of the toner appears as a pattern of light and dark patches in the resulting output image.
In view of the above, in a known image forming apparatus, instead of using a secondary transfer bias composed only of a direct current (DC) voltage, a bias in which an alternating current (AC) voltage is superimposed on a DC voltage is supplied as a secondary transfer bias, thereby preventing the pattern of light and dark. Such a secondary transfer bias causes the toner to move back and forth between the recessed portions of the recording medium and the image bearing member such as the intermediate transfer belt. Accordingly, the toner contacts the recessed portions of the recording medium, preventing improper transfer of toner. In this configuration, generation of a pattern of light and dark patches is suppressed, as compared with using the secondary transfer bias including the DC voltage only.
The superimposed bias in which the AC component is superimposed on the DC component is used as a transfer bias not only to prevent generation of the pattern of light and dark.
For example, in another known image forming apparatus, the transfer bias including the AC component superimposed on the DC component is used during double sided printing to prevent unevenness of transfer, dropouts of toner, and improper transfer of toner to a non-image formation area of a recording medium. In this configuration, a peak-to-peak voltage of the AC component is equal to or less than twice the absolute value of the DC component.
In another example, the superimposed bias including the AC component superimposed on the DC voltage is used together with an intermediate transfer member with a fluorocarbon resin surface to prevent generation of blank spots in an output image, which often happens in the intermediate transfer method. In this configuration, the peak-to-peak voltage of the AC component is equal to or greater than 2.05 times the DC component.
The superimposed bias is used also to prevent a partial toner transfer failure in character images or line images. In this case, the frequency of the AC component is equal to or less than 4 kHz and the number of cycles is 20 cycles or more.
Although advantageous and generally effective for its intended purpose, there is a drawback of using the superimposed bias with the AC component superimposed on the DC component in that the superimposed bias promotes degradation of parts such as the transfer member and shortens the life of these parts, which is not the case with the transfer bias using the DC component only.
When applying the secondary transfer bias including the superimposed bias having the peak-to-peak voltage of the AC component substantially greater than the absolute value of the DC component, for example, four times greater than the absolute value of the DC component, the toner can be transferred adequately to the recessed portions of the recording medium having a rough surface, and hence a desired image density can be obtained. The pattern of light and dark patches is prevented from appearing on the output image. Although advantageous, because the peak-to-peak voltage of the AC component of the secondary transfer bias is significantly higher, for example, four times greater than the absolute value of the DC component, the AC component promotes degradation of the transfer member.
In view of the above, there is demand for an image forming apparatus that is capable of preventing degradation of parts even when the transfer bias includes the AC component superimposed on the DC component.